The invention relates to an emission tip assembly on high-voltage electrodes for charging or discharging substrates, comprising at least one emission tip, and comprising a carrier body which is composed of an insulating material and has at least one high-resistance series resistor and is arranged on a metal profile which is provided with an insulating potting compound, wherein the at least one emission tip can be connected to a high-voltage connection by means of the series resistor. In this case, the metal profile can be connected to ground and provided with an insulating layer, and the series resistor can be arranged on the metal profile in an electrically insulated manner. The invention also relates to a method for operating an assembly of this kind in AC voltage at a specific peak voltage, and also to a method for operating an emission tip assembly with auxiliary air supply.
A large number of embodiments and variants of passive and active discharge electrodes or active charging electrodes are known. Electrodes of this kind often have a plurality of emission tips which are arranged with various grid widths in a single row, in two rows or else as a planar emission tip array in such a way that they resemble, for example, a bed of nails. Emission tips of this kind are very frequently embedded together with a current-limiting resistor in elongate U-profiles by means of insulating casting resin. The electrical resistor is associated either with each individual tip or else n tips. Passively acting discharge electrodes are often also used without current-limiting resistors in practice.
If there is an electric field, the highest possible electric field strength should be active at the tips for an assembly of emission tips of active and passive high-voltage electrodes. In addition to maintaining further boundary conditions, the respective tip has to protrude to a sufficient extent out of the insulating embedding for this purpose. This is entirely compatible with the necessarily freestanding end of a lightning conductor above the object which is to be protected.
However, for a good reason, even this condition of the freestanding tip is secondary to operating safety when electrodes of this kind are used in practice. Specifically, in order to minimize the risk of injury to machine operators by way of electrodes of this kind with emission tips, the usually rigid and solid emission tips barely protrude out of the insulating potting compound of the electrode profile. The two limbs of the usually U-shaped profile cross section are often constructed in such a way that they are level with the tips, so that the risk of injury remains minimal even in the event of unintentional lateral contact with the electrode body.
One disadvantage of this is that this physical proximity of the emission tip to the surface of the profile body under usual operating conditions considerably reduces the electric field strength at the tip since large field ranges of the electric field with a shortening free tip end increasingly pass through the insulating body to the conductive, cast conductors of the inner electrode structure and therefore do not end, as intended, at the freestanding tip in order to generate the highest possible Field strength there.
If discharge electrodes are operated in a passive manner, this is accompanied, for example, by a significant increase in the corona inception with respect to the surface potential which is to be discharged. In this case, the term corona inception describes that voltage at which free charge carriers, that is to say electrons and ions of both polarities which ultimately cause the passive discharge, are generated in front of the tips by impact ionization; the gas between the tips and the charged object surface becomes conductive. In other words, in the case of non-optimum corona inception conditions of this kind, the object surface which is to be passively discharged remains at a relatively high potential, or: the less the emission tip protrudes out of the potting compound, the lower the passive discharge capacity of the electrode.
Analogously, it is the case for actively operated discharge electrodes that, given short tips, under the explained geometric conditions, for the intended active discharging effect, the AC operating voltage of the electrode which is required for generating a sufficiently large number of air or gas ions has to be increased, as a result of which the degree of efficiency of the active discharge capacity decreases.
In this case, high operating voltages in the kilovolt range are accompanied by further disadvantages for the operation of electrodes of this kind, specifically reduced operating reliability, the disruptive proximity of machine parts which are connected to ground and, last but not least, the relatively high production costs, both of the electrode and also of the high-voltage supply unit.
For positively or negatively operated DC charging electrodes, a non-freestanding tip has the disadvantage that the charging current required for the application can flow only at relatively high operating voltage. The resulting disadvantages are comparable with those of the active discharging electrode. For the sake of completeness, it should be mentioned that this obviously also applies for special, bipolar-operated DC discharge electrodes.
DE 197 11 342 A1 discloses, for example, an active electrode which is operated with a high AC voltage, the construction of said electrode corresponding to that above. The rigid emission tips of the assembly in said document protrude only minimally out of the insulating casting resin and the two limbs of the U-shaped profile end approximately level with the tips.
Furthermore, DE 10 2011 007 138 A1 discloses a design of special high-voltage polymer resistors in connection with rigid emission tips which are used as semi-finished products when producing high-voltage electrodes.
EP 1 241 755 A2 discloses, in contrast, an active discharge electrode with air assistance. This contains even emission tips which are situated lower than the insulating surroundings of the air guide or air nozzle.
Comparably unfavorable conditions can also be found with conventional commercially available charging electrodes with and without air assistance, as are known from DE 20 2004 014 952 U1 for example.